1. Field of the Invention
The present invention relates to resistance memory elements, and particularly, to a resistance memory element including an elementary body made of a polycrystalline semiconductor ceramic.
2. Description of the Related Art
A resistance memory element includes an elementary body that can memorize resistance. This elementary body originally has, for example, a relatively high resistance. However, when at least a predetermined voltage is applied to the elementary body, the resistance is reduced and a low resistance state is maintained or memorized even after the applied voltage is removed. In contrast, when at least the predetermined voltage is applied in the opposite direction to the elementary body in a low resistance state, the resistance is increased and a high resistance state is maintained or memorized even after the voltage is removed.
Such a resistance memory element can switch between a high resistance state and a low resistance state by applying a voltage equal to or greater than a threshold in a positive direction or the opposite direction, and thus, can change and memorize the resistance. Such resistive switching enables the resistance memory element to function not only as a memory element, but also as a switching element.
A known resistance memory element is described in, for example, T. Fujii et al., “Hysteretic Current-Voltage Characteristics and Resistance Switching At An Epitaxial Oxide Schottky Junction SrRuO3/SrTi0.99Nb0.01O3”, APPLIED PHYSICS LETTERS 86, 012107 (2005). In a resistance memory element described in this published article, an interface between different types of material, more specifically, the bonded interface between a monocrystalline substrate and a SrRuO3 thin film (monocrystalline thin film), is capable of memorizing resistances. This resistance memory element has a switching voltage not greater than about 3 V, and accordingly, switches at a relatively low voltage. The switching voltage refers to a voltage at which the resistance state is changed.
Many of the circuits in which a resistance memory element may be used are operated at a rated voltage of greater than about 3 V. When a resistance memory element as described in the above published article is used as a switching element with a relatively high driving voltage, the switching voltage must be greater than the rated voltage.
However, since the switching voltage of the resistance memory element of the above published article is not greater than about 3 V, the element may switch at the driving voltage. Thus, this element cannot be used as a switching element with a relatively high driving voltage.
If a switching element that switches at a voltage of, for example, at least about 30 V is produced, another resistor must be provided in series. In this instance, the switching voltage can be set to a relatively high voltage, but the power consumption is increased due to the additional resistor. In addition, the resistor disadvantageously reduces the variation of resistances switched.
Varistors are also relevant to the present invention. For example, Japanese Patent No. 2727626 discloses a multilayer varistor including a SrTiO3 elementary body including various elements and internal electrodes made primarily of Pd in the elementary body. In order to produce such a varistor, an element that functions as an acceptor is actively diffused or added and re-oxidation is performed to form a grain boundary barrier after a reduction treatment for forming semiconductor. When at least a predetermined voltage is applied to the varistor, the varistor changes into a low resistance state, but the state is not maintained or memorized and is returned to the initial state after the applied voltage is removed. Thus, the varistor is not a resistance memory element.